This invention relates to devices for acting on webs of material in general and more particularly to an improved device using a roll of the type in which an outer hollow cylinder is braced against a crosshead by means of a fluid pressure medium.
A device for acting on webs of material comprising: at least one roll, which comprises a revolving hollow cylinder constituting the working roll circumference; a crosshead going through the cylinder with spacing on all sides, and against which the hollow cylinder is braced by means of a fluid pressure medium contained in its interior between the crosshead and the inside of the hollow cylinder in at least one chamber; force members which operate in the action plane of the roll and engage the ends of the crosshead protruding from the hollow cylinder, or corresponding points of a counter roll; and a control for holding the forces exerted by the fluid pressure medium, on the one hand, and by the force members, on the other hand, substantially in equilibrium, is known. Different embodiments of this type of device are described, for example, in German Pat. No. 14 11 327, DE-OS No. 26 23 492 and U.S. Pat. No. 3,276,102. The webs of material acted upon by such devices may be of paper, nonwoven fabric, textiles, plastic foils or similar materials and are preferably subjected to a pressure treatment by the device. In addition to pressure treatments, however, other actions may also be considered, for instance, support on support rolls of a support roll winding device.
The bending of rolls of the type including a hollow cylinder surrounding a cross-head can be controlled, so that the cylinders are capable of exerting uniform line pressure. Examples of rolls in which bending is controlled can be found in German Pat. No. 1 026 609; U.S. Pat. No. 2,395,915, and DE-AS Nos. 11 93 792; 15 61 706, and 22 30 139.
If such a roll cooperates, for instance, with a counter roll, force members for bringing the rolls together can engage either at the ends of the crosshead of the roll under consideration, which protrude from the hollow cylinder, or also at the corresponding points of the counter roll. The force members usually comprise hydraulic or pneumatic piston/cylinder units. The pressure which prevails in these piston/cylinder units must be in a given relationship to the pressure in the chamber of the hollow cylinder of the roll if equilibrium is to prevail. Otherwise, proper running of the roll is not possible; rather, if the hollow cylinder is supported on the crosshead by antifriction bearings, part of the exerted forces are absorbed by the antifriction bearings which are thereby stressed heavily. In embodiments with a hollow cylinder not supported in bearings, it is improperly displaced relative to the cross section.
Control systems have, therefore, been developed which keep the pressures, those bringing the rolls together and those within the hollow cylinder, in a certain relationship to each other. Examples for such control systems can be seen from the above-mentioned patents.
In the known embodiments, the pressure of the pison/cylinder units is used as a basis for control. This pressure does not accurately represent the forces exerted on the ends of the cylinder because the friction in the piston/cylinder unit is a factor. In the case of more stringent requirements as to the accuracy of the control, such errors are noticeable. As only one example, in a calender roll, very low line pressures must be run, in which friction errors in the rolls can lead to significant errors.
It is an object of the present invention to increase, in a device of this type, the accuracy of the distribution of the forces exerted by the force members and the force exerted by the pressure medium in the chamber.